Don't retag the shared fstring via TYPE_UCLASS in Marshal.load

This is not a real-world problem. Marshal.dump never emits this payload.
It can only be hand-crafted. Loading untrusted Marshal data is already
unsafe. This fix only avoids false positives from security scanners.

TYPE_UCLASS ('C') relabels the wrapped object's class with RBASIC_SET_CLASS.
It assumes the object was freshly allocated during load. But with
freeze: true, r_leave() interns a wrapped bare String first, and
interning a bare String returns the shared global fstring. So the relabel
hits that global object instead, corrupting every other reference to the
same interned string.

Load the wrapped object as a partial so r_leave() is deferred, and
finalize it only after the class has been set. The String is then
interned as a non-bare String, which keeps the result a distinct object
instead of the shared fstring.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

marshal.c

@@ -2012,7 +2012,11 @@ r_object_for(struct load_arg *arg, bool partial, int *ivp, VALUE klass, VALUE ex
                 hash_new_capa = rb_ident_hash_new_capa;
                 goto type_hash;
             }
-            v = r_object_for(arg, partial, 0, c, extmod, type);
+            /* Load as a partial and run r_leave() only after the class is set
+             * below: interning a bare frozen String returns the shared global
+             * fstring, and retagging that shared object would corrupt every
+             * other reference to it. */
+            v = r_object_for(arg, true, 0, c, extmod, type);
             if (RB_SPECIAL_CONST_P(v) || RB_TYPE_P(v, T_OBJECT) || RB_TYPE_P(v, T_CLASS) || RB_TYPE_P(v, T_MODULE)) {
                 goto format_error;
             }
@@ -2027,6 +2031,7 @@ r_object_for(struct load_arg *arg, bool partial, int *ivp, VALUE klass, VALUE ex
                          rb_class_name(c));
             }
             RBASIC_SET_CLASS(v, c);
+            v = r_leave(v, arg, partial);
         }
         break;
 

test/ruby/test_marshal.rb

@@ -1123,5 +1123,25 @@ class TestMarshal < Test::Unit::TestCase
       assert_same objects[1], objects[1].instance_variable_get(:@self)
       assert_same objects[0].instance_variable_get(:@self), objects[1].instance_variable_get(:@self)
     end
+
+    class StringSubclass < String
+    end
+
+    def test_uclass_fstring
+      interned = -("fstring".b)
+      assert_instance_of(String, interned)
+
+      # TYPE_UCLASS ('C') wrapping a bare (binary, no ivar) String.
+      payload = "\x04\x08C".b +
+                Marshal.dump(StringSubclass.name.to_sym)[2..] +
+                Marshal.dump("fstring".b)[2..]
+      loaded = Marshal.load(payload, freeze: true)
+
+      assert_instance_of(StringSubclass, loaded)
+      assert_predicate(loaded, :frozen?)
+      assert_not_same(interned, loaded)
+      assert_instance_of(String, interned)
+      assert_instance_of(String, -("fstring".b))
+    end
   end
 end